1. Field of the Invention
The present invention relates to circuitry for controlling distortion due to current variations in output stage driver transistors of an amplifier.
2. Background
Several factors can produce distortion in amplifiers. One of many is the output stage. Because the output stage must provide widely varying output load currents but only draw modest quiescent currents, the output and driver transistors undergo decades of current variation throughout the output drive range. The decades of current variation causes the output and driver transistor VBES to change according to logarithmic law, causing distortion when superimposed on linear signals.
A typical amplifier block diagram is shown in FIG. 1. In the amplifier of FIG. 1, an input differential voltage is applied across input stage amplifier 100 to drive a gain node 102. The gain node 102 is buffered by an output stage buffer 104 forming an output buffer for the overall amplifier of FIG. 1. The output buffer 104 has a nominal gain of +1. The frequency response is set by setting the capacitance Ccomp of capacitor 106.
Distortion created by the output buffer 104 can be modeled as a voltage source VDISTO 107 in series with the input of the output buffer 104. To correct for VDISTO, some type of feedback is typically employed to adjust the differential input voltage.
FIG. 2 shows a block diagram of an amplifier with circuitry including correction amplifier 200 to correct for VDISTO by creating an error current which is injected into the gain node 102. The gain of the correction amplifier 200, Gmxe2x80x2, allows the error from the gain node to the final output to be observed. The gain of the amplifier 200 is typically set using an internal resistor. Previous designs employed the resistor with a value set so that the magnitude of Gmxe2x80x2 was less than the input differential Gm for stability. The Gmxe2x80x2 values of previous designs were also generally flat with frequency due to the resistor R controlling Gmxe2x80x2. Note that components carried over from FIG. 1 to FIG. 2 are similarly labeled, as will be components carried over in subsequent figures.
In accordance with the present invention an improved amplifier is provided with circuitry to correct for VDISTO by creating an error current from a correction amplifier which is injected into the gain node, similar to FIG. 2. The circuit in accordance with the present invention includes components as in FIG. 2, with the correction amplifier including a correction capacitor to correct for VDISTO. Unlike typical circuits which included a resistor, with the correction capacitor the effective correction value will not be flat with frequency. The correction capacitor has a capacitance value Cxe2x80x2 preferably set equal to the capacitance of the compensation capacitor Ccomp. With the value for Cxe2x80x2 so set, Gmxe2x80x2xe2x89xa1sComp and the feedback current provided will be exactly the amount the differential input to amplifier 100 would have to be adjusted in feedback to supply the correct current to compensate for VDISTO. Thus, no input differential voltage is needed to cancel VDISTO, and the distortion is effectively cancelled. Another benefit of using the correction capacitor is that output impedance is reduced over frequency.